Carrier synchronization



De@ l, 1942-` l T. Bl. D. 'rl-ERROI` l 2,303,512

CARRIER SYNCHRONIZATION Filed June 24, 1940A 2 sheets-sheet 1 y mVENToR TESEO BRUNO ADANTE TERRONJ ATTYS.

Dec. 1, 1942. T. B. D. TERRNI CARRIER SYNCHRONIZATION 2 sheets-sheet 2 Filed June 24, 1940 .Iivewbim Teseo Bruno Dante Trram' brneys atented Dec. 1, 1942 CARRIERSYNCHROMZATION- Teseo Bruno DanteY Terron, Liverpool; England,A assignor' to Automatic Telephone & Electric Company Limited, London, England,V arrtishr company Application June.24, 1940; Serial No. 342,175 In Great Britain July 4, 1939 S'Claims. (Cl. Z50-36.)

The present invention relates to carrier telephone and like electrical signalling` systems and is more particularly concerned with. carrier frequency synchronization arrangements for. use therein.

The. general object of the invention is to provide a simple and efficient scheme for carrier frequency synchronization between a local. and distant terminal or terminals in a system of the above type. scheme is substantially independent of variations in line characteristics or of line disturbances which give rise to a detrimental modulation effect in systems involving the use-of a. pilot frequency for synchronization purposes.

According to the invention an arrangement. is provided for maintaining the carrier frequency at a number of stations in synchronism in which variations of the carrier frequency from a given value at one station cause a signalto be transmitted to the other station or stations to cause the frequency of the source of carrier waveoscillations at the other station or stationsy to be changed accordingly, the signal comprising impulses cf current and being dependent upon the relationship between the durationof" these impulses and the interval between suchpulses, e. g., on and off periods of impulses of current transmitted between the stations. It will `be understood of course that it is not essential. that no current should flow between pulses provided that such current is quite distinctive. The invention will be better understood from the followingA description of one method of carryingit .into effect reference being had to the accompanying draw-- ings, in which: y

Fig. 1 is a schematic view showing the equipment at one station at which one source of oscillations is located.

Figs. 2 and 3 are curves showing the impedance of one and two piezo crystals respectivelyr with respect to frequency for enabling an increased response to be obtained to slight variations of frequency of the source of oscillations in Fig. 1.

Fig. 4 is another schematic view showing the equipment at a second station including a second source of oscillations together with means responsive to Variations of frequency of the source of oscillations in Fig. l to vmaintain the two sources of oscillations in synchronism.

Fig. 5 is a schematic view showing in greater detail the photo-telemetering equipment.

For the purpose of this description it will be assumed that a carrier frequency system is in operation over a line between two terminal stations AS and BS. The various carrier frequencies at these stations will be derived from a common source by processes of frequency division and..multiplication in well-known manner, the

The operation of the proposed.

. which is the greater.

common source atthe master or control .station being preferably under crystal control so that the variation ofthe synchronous' carrier frequencies .will be very small.

According to the invention at the control sta'- tion AS a selected carrier frequency isarranged to control the movement ofthe pointer of a meter TM from a centralposition, that isA to'say the meter hasV a central zero. It'willbe noted that the controlling arrangement includes the source. of oscillations OSCI which feeds two parallel paths, one including the primary windingl of the transformer TI and a piezocrystal C while the -other branch includes the primary winding of a transformer T2`anda resistance RI. v Preferably the. resistance R1 is made of material whoseresistance does not vary'with temperature and in addition both the resistancev and the piezo crystal would preferably bel located inr a temperature controlled oven. The. secondary windings ofthe two transformers TI. and T2'. are connected to full wave rectiiiers consisting Yof unit rectiers MRA- MRD and MRE--MR-H' respectively, the output from these rectiersbeingconnectedin. parallel to the directv current measur-I ing instrument TM. It will be notedth'at the unit rectiers MRA to MRD are connected in the opposite direction to the unit rectii'ersMRE- MRH so that when. the currents. passing .through theA secondary windings of the transformers TI andfTZ' Vare equal no current. will `pass through the. instrument TM; If, however, ther currentsy differ then the needle of the instrument TM will. move to the right or left according as to The' crystal C is chosen to` have-a frequency of resonance which is close to but not. identical withthe carrier frequency of the source of oscillations OSCI'. The impedance curveof the crystal C is shown in Figure 2 in full line while the dotted line indicates` the frequency of'thecarrier oscillations generated by OSCI which itV will be noted cuts theV full line curve at the point P on the steep slope where the impedance changes rapidly for slight changes of frequency. Normally the resistance RIfis. adjusted so as to have substantially the same impedance. as the crystal' C'at the point P which represents the frequency it' is desired to maintain.v lit will thus be appreciated that if the frequency of the source of oscillations. shouldv change slightly the impedance of the crystalC will also change quite considerably in proportion and therefore the normal balance ofthe secondary windings of the transformers TI and T2'wil1 be disturbed and the needle of the instrument TM will move to the right or left. In place of the resistance RI a second crystal may be provided having an impedance frequency characteristic represented by the full line curve to the left in Fig. 3, the full line curve'to the right'. correspending to the curve of the crystal shown in Figure 2, from which it will be noted that the impedances of the crystals change in opposite directions for variations of frequency and therefore increase the difference between the output of the transformers TI and T2. The rectangle ing with the result that the pointer of the meter RM takes up a central yor zero position. The pointer RM is mechanically connected to a tuning lcondenser TC which is incorporated in the tuned circuit by which the frequency of the oscillations generated by the oscillator OSC2 is deterbelow the meter TM in Fig. 1 represents phototelemetering equipment PTE and generally corresponds to the transmitting apparatus described in connection with the remote control of meter readings as described in patent specication No.

2,176,720. In these arrangements as far as they concern the present invention the meter scale is continuously scanned by a beam of light which is reected back on to a photo-cell. The photocell in turn preferably controls an oscillator in the case of an alternating current signal and when the meter pointer is on the mid-scale zero position the impulses sent Vout over the line L to the distant station will have the ratio of the break period to make period equal to one. If the meter pointer is deflected from the centre or zero position then the ratio of the make and break period will be varied in accordance with the signal it is required to transmit. As described the impulses take the form of ,impulses of carrier current although it will be understood that the invention is not limited to this form of transmission as they could be impulses of voice frequency current or even impulses of direct current. At the distant station the signals pass through a filter F to an amplifier A the output of which leads to the grid circuit of a detector D by which the pulses of carrier frequency are converted into direct current pulses. In the cathode anode circuit either on the cathode or anode side of the high tension source a heavily damped meter movement RM is provided so that normally this meter would tend to take up a reading under the influence of this winding acting alone corresponding to the signal being transmitted as described in the prior specification above referred to but in this case themeter RM is provided with two windings and in order to increase the damping effect both these windings are shunted by a large capacity condenser represented by ECI and ECZ respectively preferably of the electrolytc type. The second winding of the meter RMA is normally fed with direct current from the source of oscillations provided at the station BS from which the various carrier frequencies used at that station are derived. The oscillator OSC2 which is designed to generate oscillations having the same frequency as those generated by OSCI and is of constant power feeds the primary winding of a transformer T3 via the crystal C2 which is arranged to have a frequency impedance characteristic similar to the impedance frequency characteristic shown in Figure 2, that is to say crystal C corresponds to crystal C of Figure 1. The secondary of the transformer T3 feeds a full wave rectier represented by the unit rectiers MRJ to MRM to cause a direct .current to ow through the second winding of the magnet RM in such a direction as to oppose the effect on the first winding of the current received over the line L. If the oscillator OSCI is generating at the correct frequency then the impulse ratio of the make and break periods of the impulses transmitted over the line L will be equal to one'and the current generated in the left hand winding of RM will be of such a value as to be exactly equal in its resultant effect due to'damping to the effect of the current received from the oscillator OSC2 by the right hand windmined. If the source of oscillations at OSCI varies slightly either up or down then the needle of the meter TM will move to the right or left and the ratio of the make to the break period of the impulse transmitted over the line will either become greater or less than 1, that is to say the net effect of the impulses received by the lefthand winding of the meter RM will rise or fall respectively and consequently the pointer of the meter will move so as to alter the tuning means controlling the oscillator OSC2 so that it now generates a different frequency to cause a current to be generated in the right-handwinding of RM to have an effect relative to the current in the left-hand winding to ensure that the pointer is maintained at a position at which the frequencies generated by OSCI and OSC2 are equal. It may be that the frequency at the originating station remains constant, that is to say the frequency of the oscillator OSCI, while the frequency of the oscillator OSC2 at the station BS varies. In this case the effect of such a variation will be to cause the current through the right-hand winding tovary while the effect of the current in the left-hand Winding remains constant. rIhe pointer will therefore move and the tuning condenser TC be changed to bring the frequency of the oscillator OSC2 back.

It will thus be seen that by means of the invention a very satisfactory method which is independent of line variations has been evolved for controlling the frequencies at two stations and it will be readily appreciated that the invention is applicable to more stations by linking them together with similar arrangements, that is to say, the oscillator at station A could control other stations over other lines or other channels or the oscillator at B could control the frequency at another station in a similar way to which B has been controlled by A.

It is to be noted that the receiver RM must have its pointer such that when the two currents in the right and left-hand windings respectively balance, that is to say their effects balance, then the pointer must stay in that position as that is the adjustment for correct frequency. The meter therefore in this condition would have no force exerted on it tending to make the pointer move and therefore it is important that there should be no or very little restoring force. The instrument for this purpose would therefore be constructed on the flux-meter or similar principle.

In an alternative arrangement instead of employing an indicating instrument an energy meter might be provided in which the effect of the current coil is represented by the unbalance of current obtained fromA the distant station and from thelocal source. For instance the current coil could have two windings in opposition one of which was connected to receive the impulses from the distant station and the other of which was connected to receive the rectied current from the local source of oscillations OSC2. The spindle of the meter in this case would be connected to a tuning condenser so that a change of frequency at either the sending or receiving ends would cause the vane to rotate until the changev of frequency produced synchronism. When this occurs the eects of the two currents vided that they both acted to will: balance-L and the spindle vof 'the meter will i with, the-out of balance of the two direct cur rents derived fromthe transmitted impulses and from thelocal source of voscillations at station-B- couldbe arrangedto operatelon anl iron `cored oscillator tuning coil -in such away as to directly vary the frequency-inthe fdirection required.

'I'he arrangement according to-the'invention for correcting frequency changes Vmightbe used in conjunction with an arrangement dependent upon the actual value of the frequenciesl at the two ends-soV that 'one-would either serveas a check on the oth'er'orrthe combined effect would produce a veryV fine degreer of synchronization. It will be -appreciated v.that the invention vmay be carried into elect in several-ways and is not to be considered aslimited t any precise form of receiving-equipment or to the utilisation by itself of signals'characterised by theratio ofthe duration of two distinctive Vperiods of impulses, as Vsuch an arrangement `could beV used in conjunction With other matters,-for instance `the amplitude ofthe `impulses transmitted over the line might be lconstant or they might be constant in amplitude while varying as regards the on-and oft` period or they might both vary as regards the on and off periods and instrength proproduce the desired eifect in the same direction.

It willbeunderstood that although a quartz crystal has been shown for-controlling thefrequency deviation as this gives a very satisfactory way of obtainingthis result, it will be possible at low frequencies to-makeuse' of a high' quality resonance circuit made up of -inductance coils'and condensers.'

I claim:

l. An-'arrangement for maintaining synchronism between sourcesof oscillations located at different stations, comprising a source of oscillations at one station, means for deriving from s aid source current-impulses having av variable ratio of make and break in dependence upon the frequency thereof, means for transmitting the said impulses to a distant stationf,fa second source of impulses at said ldistant station,'means for deriving a variable direct current from said second source independence upon the frequency thereof, responding means at the distant station which in response to impulses received causes a direct current to be generated correspondingto the ratio of the make `to break periods of said impulses, and a variably responsive device controlled differentially by the direct current from said responding means, and a variable direct current derived from said second source to control the frequency of said secondsource in order to maintain the oscillations of said two sources in synchronism.v

2. An arrangement for maintaining synchronism between sources of" oscillations -located at dierent stations comprising a source of oscillations at one station, means for deriving a 55 rentimpulses havingtheratibof m'ake andbreak" controlled' by` the- 'setting-Sofi said -ammeter fto v a Y variable:directcurrent- -fronii :said: sour-ce1 inf-de'- pen'denceupon .fthef frequency? thereof-,l a' ldirect idlllvaria'ble direct' current ammeter'l to i l which sa currentlis "appliedgf 'meansifor transmitting fourdistant station, a' second source ofi oscillations at-saidv distant station;- means for deriving av variable direct currentfrom -saidlsecond source y independence upon the frequency thereof, a device controlled y jointlyE by'saidimpulses and -said l derivedl variableidirectcurrent'lto control the f frequency of` said secondi source inl order to maintain the loscillations/lofsaid woisources in synchronism.k y s 3. vAnY arrangement for maintaining synchronism'between sources of oscillations located at different stations comprising a sourceofoscilla tions fat onelstation, a tuning 'device having a iffrequency" slightly different to the frequency of said source ofosoillations, means for deriving` a variable direct current from-said source controlled by said tuning device in dependence upon the variation of frequency of the source 'with' regspect to the frequency of the tuning arrangement means for transmitting said variable direct cur- Y rent to a distant station, a second source of oscillations at said distant station, a second tuning device having a frequency slightly diiferent-to the frequency of `said second source of oscillations, means for deriving-a variable direct current from said secondv source controlledbysaid second tuning device in dependenceuponthe variation of frequency-of the second source with respect to the frequency ofthe second "tuning arrangement, and afdevice-controlled jointly by said derived variable direct currents to control the frequency of said second source in order to maintain `the oscillations of said two sourcesA in synchronism.

4. An arrangement for'maintaining'synchronism between sources `of oscillations located at dilferent stations comprisinga source of oscillations at one'station, a piezo crystal having a frequency slightly dierent to the frequency of said source of oscillations,l means for deriving a variable direct current from said source controlled by said piezo -crystalin dependence upon the variation of frequency of the source with re.- spect tothe frequency of the-piezo crystal, means for transmitting said variable direct current to a distant` station, a second source of oscillations at said distant station, a second piezo crystal having a frequency slightly different to the frequency of said second source of oscillations, means for derivingfa variable direct current from said second source' controlled by said second piezo crystal in dependence uponr the variation of frequency of the second source with respect to the frequency of the second piezo crystal, and a device controlled'jointly by said derived variable direct currents to control the frequency of said second source in order to maintain the oscillations of said two sources in synchronism.

5. An arrangement for maintaining synchronism betweenV sources of oscillations located at different stations comprising a source of oscillations at one station, a piezo crystal having a frequency slightly diiferent to the frequency of said source of oscillations, a circuit including said piezo crystal and said source of oscillations, a second circuit balanced against said rst circuit and'including saidse'conwsourc'eof Aoscillations and an impedance device means for deriving dicircuit to whichdirect current'derivedzfromjsaid'- circuits is fed independence upon thedierence of currents insaidcircuits-means for transmitting a variable direct current derived Vfrom current' in said output circuit'to a distant stamaintain the oscillations of said two sources in v synchronism. l

6. An arrangemen for maintaining .synchronism between sourcesof oscillations located at different stations comprising a source of oscillations at one station, a piezo crystal having a frequency slightly different 'to-the frequency of said source of oscillations,.a circuit including said piezo crystal and said source of oscillations, an impedance device, a-second circuit including said source of oscillation and said impedance device,

means for deriving a direct current from each of said circuits, a circuit inwhich the direct currents derived from said circuits act in opposition, an ammeter in said circuit variably controlled by the out of balance currents created by a variation of frequency of the source with respect to the frequency of the piezo crystal, an` alternating current impulse generator controlled by said ammeter to varyA theA ratio of make and break periods of impulses generated in accordance'with the reading thereof as determined by a light scanning arrangement, means for transmitting said impulses of alternating current to a distant station, means for rectifying said impulses Iof alternating current at the distant station, a heavily damped meter movement having twoexciting windings, one of said windings being excited by said rectied impulses, a second source of oscillations at said distant station a second piezo crystal having a frequency slightly different to the frequency of said second source of oscillations and in circuit therewith means for deriving a variable direct current from said .circuit dependenceupon the'variation of frequency of the second source with respect to thefrequency of the second piezo crystalsa circuit enabling said variable direct current to be vfed to the second winding of said heavily damped meter movement, and tuning means controlled by said meter movement for adjusting the frequencyof said second source of oscillations whereby the frequency of the two sources may be maintained in synchronism. A

'7. An arrangement formaintaining synchronism between sources of oscillations located at different stations comprising a source of oscillations at one station, a piezo crystal having a frequency slightly below the frequency of said source of oscillations, a circuit including said piezov crystal and said source of oscillations, a second piezo crystal having a frequency slightly above the frequency of said source of oscillations a second circuit including said source of oscillation and said second piezo crystal means ,for deriving a direct current from each of said cir-A cuits, a circuit in which the direct currents de- Y `source of oscillations,- a circuit including said frequency `of the piezo crystals anl alternating current. impulse generator controlled by said ammeter to vary the ratio of make and break periods of impulses generated in accordance with the reading thereof as determined by a light scanning arrangement, means for `transmitting said impulses of alternating current to a distant station, means for rectifying said impulses of alternating` current at the distant station, a. heavily damped meter movement having two exciting windings, one of said windings being excited by said rectied impulses, a second source of oscillations at said distant station a third piezo crystal having a frequency slightly different to the frequency of said second source of oscillationl yandin circuit therewith means for deriving a variable direct current from said circuit in dependence upon the variation of frequency of the second source lwith respect to the-frequency of the thirdl piezo crystal, a` circuit enabling said vari- ,able direct current to be fedy to the second winding of said heavily damped meter movement, and

8. An arrangement for maintaining synchro-4 nismrbetween sourcesof oscillations located at different stations comprising a source of oscillations at one station, a piezo crystal having al frequency slightly different to the frequency of said piezo crystal and said source of oscillations, an impedance device a second circuit including said source Aofbscillation and said'impedance device means for deriving a direct current from each of said circuits, a circuitl in which the direct currents derived from said circuits-act in opposition an ammeter in saidl circuit variably controlled by the out of balance currents created by a variation lof frequency of the sourcey withrespect to the.4 frequency of the piezo crystal anA alternatingy current impulse generator controlled by said ammeter to vary the ratio of make and break periods of impulses generated in accordance with the reading thereof as f determined by alight scanning arrangement, means for transmitting said impulses of alternating current' to a distant station, means for rectifyingsaid impulses of alternating current at the distant station, an electromagnetic device having two exciting windings, oneof said windings being excited by said rectified impulses, a second -source of oscillations at said` distant station, asecond piezo crystal having a frequency slightly different to thefrequency-of said second source of oscillations and incircuit therewith Vmeans yfor deriving a variable directcurrent from saidcircuit infdependence upon the Variation Aof frequencyoffthe-second source y with respect' to thefrequency of the second piezcv, crystaLfa circuit enabling said variable direct current to be fed to thesecond winding of saidelectromagnetic device, and tuning means controlledby saidv electromagnetic Vdevice for adjusting thefrequency of saidsecond source of oscillations whereby the ifrequency yof j the two sources may be maintained in synchronism.

'russo BRUNO DANrvrEiaRoNI. 

